The present invention relates to an antireflection film effective for improving the transparency of an image display device such as a liquid crystal display (LCD), a plasma display (PDP), CRT, EL or a touch panel, or an optical product made of glass, such as a lens for eye glasses, and an antireflection glass. Particularly, it relates to an antireflection glass excellent in mass productivity.
A transparent substrate such as a glass substrate provided with transparent electrodes, to be used for an image display device represented by LCD, PDP, CRT, EL or a touch panel, generates a reflected light of about 4% on one side thereof, which causes deterioration in the visibility or the transmittance. Therefore, for the purpose of improving the visibility or the transmittance by reducing the amount of light reflected from the substrate, a method of forming a so-called antireflection film such as a multi-layer film prepared by laminating thin films having low reflective indices or thin films having different refractive indices, on the substrate surface, has been employed.
In general, with an antireflection film of a multi-layer structure, effective antireflection can be realized within a wide wavelength region, but it is poor in mass productivity since a technique for controlling the film thickness of each layer with high precision is required. Therefore, some thin films having low refractive indices have been proposed which can be formed by a coating method as a method for forming an antireflection film simply and conveniently.
JP-A-6-157076 proposes to prepare an antireflection film having a low refractive index by forming fine irregularities on the surface of a coating film by using, as a coating liquid, a mixture of hydrolytic condensates of alkoxysilans having different molecular weights. However, there has been a problem such that control of the irregularities on the coating film surface by controlling the relative humidity at the time of forming the coating film is difficult, or the production of the condensates having different molecular weights is conversant.
JP-A-5-105424 discloses a method of employing a coating liquid containing fine particles of MgF2, but there has been a problem such that the formed coating film is poor in the mechanical strength and the adhesion to the substrate, and is further inferior in the antireflection performance.
The present inventors have previously found that by heat-treating a coating film obtained from a polysiloxane solution employing a fluoroalkylsilane, at a temperature of from 80 to 450xc2x0 C., a coating film having a low refractive index and a large contact angle of water can be formed (U.S. Pat. No. 5,800,926; JP-A-9-208898). When such a coating film is formed on the surface of a display device, the large contact angle of water is a useful property as an additional function, but when it is used in the interior of the device as a highly transparent substrate, it will be essential to form another film on the surface of the coating film, and in such a case, the large contact angle of water is detrimental to the film forming on the coating film.
Therefore, a further study has been made, and as a result, it has been found that by optimizing the temperature of the heat treatment, it is possible to obtain a coating film having a small contact angle of water and a low refractive index.
Namely, it is an object of the present invention to provide an antireflection film which has a small contact angle of water and which is excellent in the antireflection performance, by a method which is capable of treating at a low cost, in a large amount and over a large area.
In a first aspect, the present invention provides a process for forming an antireflection film as adhered on a glass surface, which comprises preparing a reaction mixture comprising a silicon compound (A) of the following formula (1):
Si(OR)4 xe2x80x83xe2x80x83(1) 
wherein R is a C1-5 alkyl group, a silicon compound (B) of the following formula (2):
R1Si(OR2)3 xe2x80x83xe2x80x83(2) 
wherein R1 is a C1-18 organic group, and R2 is a C1-5 alkyl group, an alcohol (C) of the following formula (3):
R3CH2OH xe2x80x83xe2x80x83(3) 
wherein R3 is a hydrogen atom or an unsubstituted or substituted C1-12 alkyl group, and oxalic acid (D), in a ratio of from 0.05 to 4.5 mol of the silicon compound (B) per mol of the silicon compound (A), in a ratio of from 0.5 to 100 mol of the alcohol (C) per mol of the total alkoxy groups contained in the silicon compounds (A) and (B), and in a ratio of from 0.2 to 2 mol of the oxalic acid per mol of the total alkoxy groups contained in the silicon compounds (A) and (B); heating the reaction mixture at a temperature of from 50 to 180xc2x0 C. until the total amount of the silicon compounds (A) and (B) remaining in the reaction mixture becomes at most 5 mol %, while maintaining a SiO2 concentration of from 0.5 to 10 wt % as calculated from silicon atoms in the reaction mixture and maintaining absence of water, to form a polysiloxane solution; coating the polysiloxane solution on a glass surface to form a coating film; and heat-curing the coating film at a temperature of from 480 to 520xc2x0 C.
In a second aspect, the present invention provides the process for forming an antireflection film according to the first aspect, wherein in the formula (2) for the silicon compound (B), the organic group represented by R1 contains fluorine atoms.
In a third aspect, the present invention provides the process for forming an antireflection film according to the first aspect, wherein the formula (2) represents a silicon compound (B) of the following formula (4):
CF3(CF2)nCH2CH2Si(OR4)3xe2x80x83xe2x80x83(4)
wherein n is an integer of from 0 to 12, and R4 is a C1-5 alkyl group.
In a fourth aspect, the present invention provides an antireflection film having a refractive index of from 1.33 to 1.38 and a contact angle of water of at most 40xc2x0, which is formed as adhered on a glass surface, by preparing a reaction mixture comprising a silicon compound (A) of the following formula (1):
Si(OR)4xe2x80x83xe2x80x83(1)
wherein R is a C1-5 alkyl group, a silicon compound (B) of the following formula (2):
R1Si(OR2)3xe2x80x83xe2x80x83(2)
wherein R1 is a C1-18 organic group, and R2 is a C1-5 alkyl group, an alcohol (C) of the following formula (3):
R3CH2OHxe2x80x83xe2x80x83(3)
wherein R3is a hydrogen atom or an unsubstituted or substituted C1-12 alkyl group, and oxalic acid (D), in a ratio of from 0.05 to 4.5 mol of the silicon compound (B) per mol of the silicon compound (A), in a ratio of from 0.5 to 100 mol of the alcohol (C) per mol of the total alkoxy groups contained in the silicon compounds (A) and (B), and in a ratio of from 0.2 to 2 mol of the oxalic acid per mol of the total alkoxy groups contained in the silicon compounds (A) and (B); heating the reaction mixture at a temperature of from 50 to 180xc2x0 C. until the total amount of the silicon compounds (A) and (B) remaining in the reaction mixture becomes at most 5 mol %, while maintaining a SiO2 concentration of from 0.5 to 10 wt % as calculated from silicon atoms in the reaction mixture and maintaining absence of water, to form a polysiloxane solution; coating the polysiloxane solution on a glass surface to form a coating film; and heat-curing the coating film at a temperature of from 480 to 520xc2x0 C.
In a fifth aspect, the present invention provides the antireflection film according to the fourth aspect, wherein in the formula (2) for the silicon compound (B), the organic group represented by R1 contains fluorine atoms.
In a sixth aspect, the present invention provides the antireflection film according to the fourth aspect, wherein the formula (2) represents a silicon compound (B) of the following formula (4):
CF3(CF2)nCH2CH2Si(OR4)3xe2x80x83xe2x80x83(4)
wherein n is an integer of from 0 to 12, and R4 is a C1-5 alkyl group.
In a seventh aspect, the present invention provides an antireflection glass comprising a glass and the antireflection film as defined in the fourth aspect, formed on one side or both sides of the glass.
Now, the present invention will be described in detail with reference to the preferred embodiments.
Examples of the alkyl group R in the above formula (1) include methyl, ethyl, propyl, butyl and pentyl. Preferred examples of the silicon compound (A) include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane and tetrabutoxysilane. Among them, particularly preferred are tetramethoxysilane and tetraethoxysilane.
Examples of the alkyl group R2 in the above formula (2) include, methyl, ethyl, propyl, butyl and pentyl. Preferred examples of the silicon compound (B) include methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, propyltrimethoxysilane, propyltriethoxysilane, butyltrimethoxysilane, butyltriethoxysilane, pentyltrimethoxysilane, pentyltriethoxysilane, heptyltrimethoxysilane, heptyltriethoxysilane, octyltrimethoxysilane, octyltriethoxysilane, dodecyltrimethoxysilane, dodecyltriethoxysilane, hexadecyltrimethoxysilane, hexadecyltriethoxysilane, octadecyltrimethoxysilane, octadecyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, xcex3-aminopropyltrimethoxysilane, xcex3-aminopropyltriethoxysilane, xcex3-glycidoxypropyltrimethoxysilane, xcex3-glycidoxypropyltriethoxysilane, xcex3-methacryloxypropyltrimethoxysilane, xcex3-methacryloxypropyltriethoxysilane, trifluoropropyltrimethoxysilane, trifluoropropyltriethoxysilane, tridecafluorooctyltrimethoxysilane, tridecafluorooctyltriethoxysilane, heptadecafluorodecyltrimethoxysilane and heptadecafluorodecyltriethoxysilane. These compounds may be used alone or in combination as a mixture of two or more of them.
Among such preferred silicon compounds (B), particularly preferred is a fluorine-containing silane of the following Formula (4):
CF3(CF2)nCH2CH2Si(OR4)3xe2x80x83xe2x80x83(4)
wherein n is an integer of from 0 to 12, and R4 is a C1-5 alkyl group, such as trifluoropropyltrimethoxysilane (n=0), trifluoropropyltriethoxysilane (n=0), tridecafluorooctyltrimethoxysilane (n=5), tridecafluorooctyltriethoxysilane (n=5), heptadecafluorodecyltrimethoxysilane (n=7) or heptadecafluorodecyltriethoxysilane (n=7).
Examples of the unsubstituted alkyl group R3 in the above formula (3) include methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl and octyl. Examples of the substituted alkyl group R3 include hydroxymethyl, methoxymethyl, ethoxymethyl, hydroxyethyl, methoxyethyl and ethoxyethyl.
Preferred examples of the alcohol (C) include methanol, ethanol, propanol, n-butanol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether and propylene glycol monoethyl ether. These compounds may be used alone or in combination as a mixture of two or more of them. Among them, particularly preferred is ethanol.
The content of the silicon compound (B) is preferably from 0.05 to 4.5 mol per mol of the silicon compound (A). If the content is less than 0.05 mol, it tends to be difficult to form a coating film having a refractive index of at most 1.40, and if it exceeds 4.5 mol, it tends to be difficult to obtain a uniform solution.
The content of the alcohol (C) is preferably from 0.5 to 100 mol, particularly preferably from 1 to 50 mol, per mol of the total alkoxy groups contained in the silicon compounds (A) and (B). If the content is less than 0.5 mol, it takes a long time to form the polysiloxane, and it tends to be difficult to form a coating film having high hardness from the liquid containing the polysiloxane thereby obtained. On the other hand, if it exceeds 100 mol, the SiO2 concentration in the obtained polysiloxane-containing liquid tends to be inadequate, and concentration will be required prior to coating, such being inefficient.
The content of oxalic acid (D) is preferably from 0.2 to 2 mol, particularly preferably from 0.25 to 1 mol, per mol of the total alkoxy group contained in the silicon compounds (A) and (B). If it is less than 0.2 mol, it tends to be difficult to form a coating film having high hardness from the obtained solution, and if it exceeds 2 mol, the mixture will contain a relatively large amount of oxalic acid (D), whereby it tends to be difficult to obtain a coating film having the desired performance.
The reaction mixture comprising the silicon compounds (A) and (B), the alcohol (C) and the oxalic acid (D), may be formed by mixing such components. This reaction mixture is preferably heated in the form of a solution. For example, it is preferably heated as a reaction mixture in the form of a solution obtained by preliminarily adding the oxalic acid (D) to the alcohol (C) to form an alcohol solution of oxalic acid and then mixing the silicon compounds (A) and (B) thereto. This heating can be carried out at a liquid temperature of from 50 to 180xc2x0 C. and preferably carried out, for example, in a closed container or under reflux, so that no evaporation or volatilization of the liquid occurs.
If the liquid temperature at the time of forming the polysiloxane solution is lower than 50xc2x0 C., the solution tends to have turbidity or tends to contain insoluble substances, whereby it tends to be a non-uniform solution. Therefore, the liquid temperature is preferably at least 50xc2x0 C., and as the temperature is high, the operation can be completed in a short period of time.
However, heating at a temperature higher than 180xc2x0 C. is inefficient without bringing about any additional merit. The heating time is not particularly limited. For example, it is usually about 8 hours at 50xc2x0 C. and about 3 hours under reflux at 78xc2x0 C. Usually, the heating is terminated when the amount of the remaining silicon compounds (A) and (B) becomes at most 5 mol %, based on the total charged amount of the silicon compounds (A) and (B). If the remaining amount exceeds 5 mol %, when such a solution is coated on a substrate surface and heat-cured, the resulting coating film tends to have pinholes, or it tends to be difficult to obtain a coating film having adequate hardness.
The polysiloxane solution obtained by the above heating, may be concentrated or diluted, as the case requires. At that time, the SiO2 concentration calculated as a solid content in the polysiloxane solution is preferably from 0.5 to 15 wt %. If the SiO2 concentration is lower than 0.5 wt %, it tends to be difficult to obtain a desired thickness by a single coating operation, and if it exceeds 15 wt %, the pot life of the solution tends to be inadequate.
The prepared solution is coated and heat-cured to obtain a desired coating film, by a coating method which is commonly employed. The coating film formed on the substrate (glass) may be heat-cured as it is, but before the heat-curing, it may be dried at a temperature of from room temperature to 120xc2x0 C., preferably from 50 to 100xc2x0 C. and then heated at a temperature of from 480 to 520xc2x0 C. If the heating temperature is lower than 480xc2x0 C., the contact angle of water exceeds 40xc2x0, such being undesirable. On the other hand, if it exceeds 520xc2x0 C., the refractive index tends to increase beyond 1.38, whereby the coating film tends to be poor in the anti-reflection performance. Accordingly, the above-mentioned temperature range is proper in order to form a coating film which has a small contact angle of water at a level of at most 40xc2x0 and a refractive index of from 1.33 to 1.38 and which is excellent in the anti-reflection performance.
The time for this heating is not particularly limited, but it is usually from 5 to 60 minutes. Such heating can be carried out by a conventional method, for example, by using a hot plate, an oven or a belt furnace.
As the above coating method, a conventional method such as a spin coating method, a dip coating method, a roll coating method or a flexoprinting method, may, for example, be usually employed.